The present invention relates to an apparatus for the purification of a constituent gas and also to the generation of a constituent gas and the subsequent separation and purification from a mixed gas flow. More specifically, the present invention relates to the generation or purification of hydrogen from a mixture containing hydrogen. The apparatus utilizes one or more gas extraction membrane for removing hydrogen or other extractable gas from a mixed gas flow.
A common technology for hydrogen generation involves the following processes in series: high temperature steam reforming at pressures between 150 and 300 psi, a high temperature water-gas shift reactor, a low temperature water-gas shift reactor, and a hydrogen purifier. The purifier is most typically a PSA (pressure swing absorption) system, but can also be a membrane or partial oxidation process. The water-gas shift reactors are employed to generate additional hydrogen from carbon monoxide produced in the high temperature reformer via the following (water-gas shift) reaction:
CO+H2Oxe2x86x92CO2+H2.
Cooling is required because the water-gas shift reaction is exothermic and equilibrium limited. Thus, lowering the gas temperature promotes the reaction. There is a limit to the effectiveness of lower temperatures, since the reaction rate generally decreases with decreasing temperature. The high temperature shift reactor is thus designed to operate at an intermediate temperature, lower than that of the reformer but higher than that of the low temperature shift reactor that follows. The high temperature shift reactor generates hydrogen at relatively high catalyst turnover, but gas temperature in this reactor limits the amount of hydrogen that can be produced. Inter-cooling between the two shift reactors reduces this temperature so that additional hydrogen can be produced, albeit at a lower catalyst turnover. A single stage adiabatic reactor (membrane or otherwise) could be used with a sufficiently low feed temperature, but the catalyst effectiveness would suffer because of the low temperature at the entrance. Further, heat generated by the water-gas reaction would raise the temperature in the reactor, limiting the effectiveness of this approach.
A common technology for extracting pure hydrogen from industrial streams, such as for hydrogenation for changing the balance of hydrogen in those streams or to increase reaction selectivity, is to use membranes of palladium or palladium alloys alone or supported structurally by a matrix. Membranes which contain thick enough palladium layers to be made without holes and not break during service tend to be expensive and have relatively high resistance to hydrogen permeation. The membranes are disposed in a housing. A mixed gas flow is conducted to the housing wherein the extraction occurs. Extracted gas (such as hydrogen) is preferentially extracted through the membranes and exits through an outlet port. A second outlet allows for the exhaust of raffinate out of the chamber. Examples of such chambers are shown in U.S. Pat. Nos. 5,205,841 and 4,468,235.
Several membrane variations and module designs have been proposed to minimize this effect. Membranes can include porous ceramics either by themselves or coated with palladium alloys or with silica and palladium coated refractory metals and alloys, especially those based on Nb, V, Ta, Ti, Zr. These have greater strength than palladium and palladium-based alloys, are cheaper per unit volume, and most have greater intrinsic permeabilities to hydrogen. Although the alternatives are less expensive than Pd, they are not less expensive compared to polymers. Thus, with all of these membranes more attention must be directed to module designs that make efficient use of the membrane surface and provide a high recovery percentage without undue gas-phase mass transfer resistance. To date, no commercial module has been described that is particularly efficient for large scale hydrogen extraction using any of these membranes.
An example of an apparatus for hydrogen separation is disclosed in U.S. Pat. No.4,468,235 to Hill (Hill ""235). The Hill apparatus for separating hydrogen from fluids and includes, mounted axially in a cylindrical pressure vessel, a plurality of membranes in the form of tubes coated on either the inside or the outside or both sides with coatings having a high permeability to hydrogen. There is also a fluid flow inlet and a raffinate flow outlet and a header to collect hydrogen. No sizes or criticalities are disclosed for the extraction membrane. Additionally, since this design provides no mechanism for flow distribution or turbulence generation, the separation efficiency of this apparatus is not maximized.
Another example of a similar apparatus for hydrogen extraction is disclosed in U.S. Pat. No.5,205,841 to Vaiman (Vaiman ""841) issued Apr. 27, 1993. The Vaiman ""841 patent discloses an apparatus for separating hydrogen from gas and gas liquid mixtures at low temperature. The Vaiman ""841 apparatus includes a plurality of axially mounted tubes coated on both their inside and outside surfaces with palladium/platinum black. There is also a fluid flow inlet and a raffinate flow outlet and a header to collect hydrogen. Vaiman ""841 does not teach any sizes or criticalities for the extraction membrane or its arrangement within the structure. Additionally, as similarly stated above regarding the Hill patent, the Vaiman ""841 design provides no mechanism for flow distribution or turbulence generation. Separation efficiency of this apparatus is not maximized.
Another typical design for large hydrogen extractors uses tubular membranes of palladium-silver alloy in spiral form. This tubing generally has an outer diameter of 0.0625 to 0.125 inches and wall thickness of approximately 0.003 inches. For the smaller diameter tubes, the source hydrogen flows over the outside of several wound helixes made from 10 to 15 feet of tubing. These hydrogen extractors typically require complex expensive construction that limits heat and mass transport. Also, since pressure drops become excessive when the tube length exceeds about 25 feet, large modules end up with 40 or more nested and stacked helixes that must be hand assembled in a large tubular bundle without damaging any single one of the delicate tubes. This is a delicate construction process by any standard.
Large diameter tubes avoid maldistribution and assembly problems by driving all of the flow through a single tube. The practical limit is reached at about 100 feet. Longer lengths lead to destructive harmonic vibrations, especially during start-up and shut-down. Also, since module size increases with the square of the tube diameter, such units have had to be too big to site comfortably. Further, temperature uniformity is even harder to maintain than with {fraction (1/16)} inch units.
The spiral type designs are particularly difficult to form when dealing with coated refractory metals or with ceramics, as these materials are more brittle than palladium and coated membranes require more gentle handling than homogenous palladium alloys. The spiral type designs inherently have problems with scraping of the membrane surfaces and with kinking of the tubing material during manufacture thereby leading to inherent weaknesses in the tubes which are utilized under pressure. For large scale applications, these spiral-type hydrogen extractors tend to be larger in overall size than the module of the present invention thereby adding to the cost of the structure, sitting, shipping, maintenance, manufacture, and making them unpleasant to the eye.
To date, modules based on tubular ceramics or ceramic-based membranes known are based on a single pair of concentric tubes. The diameter of the ceramic membranes is approximately 0.375 inches. Such designs cannot be readily scaled up for commercial applications.
The present invention provides a hydrogen extraction module which eliminates the spiral-type extraction membranes and is much more simple to construct, more compact, and can be more easily constructed from difficult materials, such as ceramics, and from high diameter to wall ratio metal tubes.
The present invention also provides improved hydrogen recovery from relatively impure mixtures through the use of critically sized extraction membranes and turbulence generating bumps or packing.
Another approach to the problems of palladium based membranes recognizes that the specific alloys are chosen by a trade-off between cycling stability, ease of drawing, high permeance, lower volumetric cost, and relatively good surface properties. Currently, the single material that most closely meets all of these criteria is made from palladium-silver alloys containing 23 to 25% silver. These tubes typically trade off exposure for moderate cycling stability. They typically do not break for about two years in operation and have moderate drawability against their relatively high expense and high resistance to hydrogen permeation, especially at temperatures below 300xc2x0 C. and for gas streams containing sulfur, carbon monoxide, and olefins.
Several options to palladium-silver membranes have been suggested, but are not in common use. For example, the British Patent No. 1,292,025 to Darling discloses a membrane requiring porous or discontinuous palladium coat over a base of refractory metal Nb, V, or Ta. The U.S. Pat. No. 4,496,373 to Bohr et al. discloses alloying the palladium layer with silver, calcium or yttrium. The patent also requires an intermediate melt layer. The U.S. Pat. No. 4,536,196 to Harris discloses essentially a palladium membrane which is coated with various metals as poisons to prevent the fouling of the palladium surface. Under some circumstances, this poisoning can be advantageous to the surface properties of the membrane, but the high cost and low reliability of palladium remains. The U.S. Pat. No. 4,313,013 to Harris shows similar palladium membranes that have been in use.
The U.S. Pat. No. 3,350,846 to Makrides et al. discloses a process of purification of hydrogen by diffusion through a very thin membrane of palladium coated Group Vxe2x80x94B metal.
The U.S. Pat. No. 5,215,729 (""729) issued Jun. 1, 1993 to the inventor of the present application and incorporated herein by reference teaches membranes which combine the strength and high permeation of refractory metals with a coating of palladium or palladium alloys to improve the surface properties of the membranes. As with single-layer palladium alloys, selectivity is essentially 100% for hydrogen extracted. Applicant has observed that some of the best refractory metals can be difficult to fabricate into tubes or modules. Applicant has further observed that the surface properties of some of these membranes were often far better than those of single layer palladium-silver, especially at low temperatures and in the presence of carbon monoxide, hydrogen sulfide, and olefins.
In view of the above, a further object of the present invention is to improve on the properties of palladium-silver and similar alloys by adding a coating of palladium or similar materials to improve the surface properties. The resulting membranes have good strength, ease of fabrication, good durability, relatively low resistance to hydrogen even at low temperatures, improved resistance to carbon monoxide, H2S and olefins, fair resistance to embrittlement, and a hydrogen selectivity that can exceed that for palladium-silver because the operating temperature can be lower.
A membrane reactor is detailed herein including a chamber, an inlet for introducing a mixed gas stream into the chamber, and a gas separator membrane having a surface in fluid communication with the mixed gas stream. The opposing surface of the gas separation membrane is in contact with a constituent gas of the mixed gas stream that has passed through the membrane that is disposed within the dimensions of the chamber. A catalyst is disposed within the chamber that facilitates an exothermic chemical reaction of the mixed gas stream. The membrane reactor is equipped with a first outlet in fluid communication with the opposing surface of the membrane for removing the constituent gas that has passed through the membrane and a second outlet for removing a waste gas stream from the chamber that has been depleted in the constituent gas. The membrane reactor is further equipped with a flowing coolant in thermal contact with the chamber for withdrawing thermal energy from the chamber.
A process for separating a gas component from a mixed gas stream includes reacting the mixed gas stream exothermically with a catalyst in the presence of a membrane selected for passing the gas component and flowing a coolant in thermal communication with the mixed gas stream. A method of operating a water-gas shift membrane reactor according to the present invention includes flowing a coolant in thermal communication with the reactor to withdraw heat generated by the water-gas shift reaction.